Precision grinding operations remove metal from an article at a moderately high rate to achieve a precisely shaped finished article having a specified size and surface quality. Typical examples of precision grinding include finishing bearing components and machining engine parts to fine tolerances. Coolants and lubricants frequently are used to improve the efficiency of precision grinding metal parts.
A "wet" method of cooling and lubricating involves bathing the grinding zone continuously during cutting with copious quantities of low temperature, fresh or recirculating liquid. Typically, the liquid is an aqueous composition containing minor concentrations of process aids. The liquid lowers grinding zone temperature to protect the tool and work piece from thermal degradation. It also flushes the tool to carry away swarf which might otherwise dull the abrasive if permitted to fill voids between abrasive particles or weld onto the particle surfaces.
There are numerous drawbacks to wet grinding. To name a few, the process is messy to operate; the liquid must be recovered for reuse or discarded in an environmentally sound manner; the presence of process aids contributes to the difficulty of recovery and adds to operating cost; the aqueous liquid can corrode parts of the grinding machinery; and the liquid is unpleasant to work with in a very cold, ambient environment.
Precision grinding also can be accomplished by a "dry" method. No flushing flow of liquid is externally applied to the grinding zone. To dry grind thermally-sensitive or difficult to grind metals, such as stainless steel, it remains desirable to lubricate the grinding zone. To accomplish this lubrication, lubricant traditionally has been supplied to the local grinding site by periodic application of solid lubricant to the face of the grinding tool, or by filling the pores of suitable abrasive such as those in vitreous abrasive tools with selected additives. Chemicals, such as sulfur, and other lubricating fillers have been used. These additives reduce loading and glazing of the abrasive, make the tool more free-cutting and reduce the incidence of burn. The additives are usually added to the abrasive after firing the bond to prevent thermal degradation of the additives and to permit proper formation of the abrasive during tool fabrication.
Dry grinding provides the advantageous feature that very little lubricant is consumed because the lubricant is deposited directly into the grinding zone. Moreover, the lubricant need not be water soluble because it is not brought to the grinding zone in cooling water. Unfortunately, additives placed in the pores, especially low viscosity liquids, are not retained in the abrasive tool for long duration. They tend to distribute unevenly in the wheel after long periods of standing, and they can partially or completely seep out of the wheel over time. In the important application of dry precision grinding using abrasive wheels operated at high speed, centrifugal force tends to expel pore-resident low viscosity liquid additives. The expelled additives splatter the work area and deplete the amount of additives available at the grinding site to aid grinding. It is desirable to provide vitreous bonded abrasive wheels which are loaded with uniformly distributed concentrations of predominantly low viscosity lubricants and which can deliver such lubricants to the grinding site over the full life of the abrasive.
Various materials have been suggested as additives for porous abrasive tools to improve grinding performance. Paraffin wax is an example of such a material. See, e.g., U.S. Pat. No. 1,325,503 to Katzenstein. Paraffin wax becomes tacky at a relatively low temperature and tends to cause loading of the face of the grinding wheel, an undesirable characteristic in precision grinding processes. A stearic acid material was reported to be superior to paraffin wax in: A. Kobayashi, et al, Annals of the C.I.R.P., Vol. XIII, pp. 425-432, 1966.
U.S. Pat. No. 4,190,986 to Kunimasa teaches an improvement in grinding efficiency and a reduction in workpiece burn may be achieved by the addition of a heated mixture of higher aliphatic acids and higher alcohols to the pores of resin bonded grindstones. The patent discloses that, unlike resin bonded tools, vitrified bonded tools do not show an improvement in grinding efficiency. In vitrified bond tools the additive is reported to function only as a lubricant, and was not observed to improve grinding efficiency.
U.S. Pat. No. 3,502,453 to Baratto discloses resin bonded abrasive tools containing hollow spheres filled with lubricant, such as SAE 20 oil encapsulated in a urea-formaldehyde capsule. Graphite is used in the resin bonded superabrasive tools disclosed in U.S. Pat. No. 3,664,8 19 to Sioui. Graphite improves grinding efficiency and lubricates the workpiece during dry grinding operations.
U.S. Pat. No. 4,239,501 to Wirth teaches the application to the cutting surface of an abrasive tool of a combination of sodium nitrite and a wax, such as paraffin, cerate and stearic acid or microcrystalline waxes.
Sulfur is known to be an excellent lubricant for precision grinding of metal parts. In M.A. Younis, et al, Transactions of the CSME, Vol. 9, No. 1, pp. 39-44, 1985, sulfur was reported to be superior to waxes and varnishes as a grinding aid impregnated into grinding tools. However, previous attempts to use sulfur-loaded tools, particularly high rotational speed abrasive wheels, have been problematic. Because of combustion at the grinding temperatures, sulfur-containing abrasive tools are used only in wet grinding processes. Often after only brief operation, centrifugal force tends to redistribute sulfur within a grinding wheel. Because sulfur has a relatively high density, the wheel may becomes unbalanced, start to chatter, and become unusable for precision grinding.
Sulfurized cutting oils have been used as an alternative to sulfur impregnated abrasive grinding wheels in order to avoid balance problems, but the oils generally have low viscosity. Therefore, abrasive wheels loaded with such oils suffer from the drawbacks discussed above.
Wet grinding is the preferred way to precision grind at high speed when employing sulfur-based process aids. The sulfur is normally used in the form of a water soluble or dispersible, low viscosity metal cutting oil which is mixed with the coolant. This is a very inefficient use of sulfur because an excess amount of sulfurized oil must be added to the large volume of liquid coolant. Sulfur also is an environmental contaminant and spent coolant must be treated to remove sulfurized materials before disposal.
Thus, none of the prior art grinding additives has been entirely satisfactory for use in vitrified bonded abrasive tools for precision grinding operations, particularly as the environmental effects of sulfur and other active grinding aids become more difficult to manage.
The need for improved grinding aids for precision grinding operations became even more acute with the introduction of sintered sol gel alumina abrasive grains during the 1980s. Abrasive tools comprising seeded or unseeded sintered sol gel alumina abrasive grain, also referred to microcrystalline alpha-alumina (MCA) abrasive grain, are known to provide superior grinding performance on a variety of materials. The manufacture, characteristics and performance of these MCA grains in various applications are described in, for example, U.S. Pat. Nos. 4,623,364, 4,314,827, 4,744,802, 4,898,597 and 4,543,107, the contents of which are hereby incorporated by reference.
The MCA grain morphology is designed to cause microfracture of the grain particles during grinding. The microfracture capability prolongs the life of the abrasive grain by wearing away each grain particle a portion at a time rather than dislodging a whole particle. It also exposes fresh abrasive surfaces, in effect causing the abrasive to self-sharpen during grinding. Because of its extraordinary sharpness relative to other abrasive grains, the MCA grain is characterized by the ability to cut with a minimum amount of grinding energy when it is used for dry grinding processes employing a vitrified bonded tool. The threshold power needed to initiate dry grinding with MCA grain is essentially zero. Under wet grinding conditions utilizing a water-based coolant, the MCA grain does not perform as well with respect to the amount of power needed to initiate grinding. Because many precision grinding operations cannot tolerate dry grinding processes, even with MCA grain, it has been necessary to develop a lubricant component that is effective as a coolant and grinding aid for vitrified bonded abrasive tools containing MCA grain. The lubricant component of the invention is effective with MCA grains in either wet or dry grinding processes.